Crystal oscillators find many applications in systems requiring a fixed frequency oscillating signal. The Pierce oscillator as described, for example, in U.S. Pat. Nos. 3,725,822 and 3,935,546 and hereinafter in the detailed description of the prior art section is a common variety of CMOS crystal controlled oscillator. The Pierce oscillator includes a quartz crystal operating in parallel with an inverting amplifier which oscillates at the resonant frequency of the crystal. One noted problem with the conventional Pierce oscillator is the high operating current drawn through the inverting amplifier which increases exponentially with the power supply voltage. Reducing power consumption is an important criteria especially in battery driven applications such as wristwatches. Furthermore, the quartz crystal should be excited with a mono-frequency signal operating at its resonant frequency. Unfortunately, the Pierce oscillator generates a square output waveform containing multiple harmonics of the fundamental frequency which may interfere with the crystal's inherent resonant vibrating rhythm.
Hence, what is needed is an improved crystal oscillator having reduced operating current and producing a mono-frequency output signal at the fundamental frequency of the quartz crystal.